The long-term goal of this project is to define the mechanisms whereby mesoderm cells become committed to undergo myogenesis. The focus of this proposal is to use an immunological approach to study the role of cell surface interactions in mesodermal differentiation. Fluorescent antibodies will be used to identify subpopulations of cells present within the segmental plate mesoderm and somites and to follow their pathways of migration and maturational changes during the development of the chick embryo. The antibodies also will be used to isolate and analyze cell surface molecules by immunoprecipitation, polyacrylamide gel electrophoresis and Western blotting. The involvement of selected cell surface molecules in the processes of adhesion, replication, cell-cell recognition, differentiation and fusion will be analyzed by culturing mesoderm cells in the presence of the antibodies. Subpopulations of cells will be purified by fluorescence-activated cell sorting and their developmental potential determined in vivo, after grafting the cells into myogenic and chondrogenic regions of the somite, and in vitro, under conditions which promote myogenesis or chondrogenesis. These studies will lay the groundwork for in depth biochemical and genetic analyses of embryonic mesoderm cells and their derivatives, the myogenic and chondrogenic precursors. Characterizations of embryonic precursor cells at the cellular and biochemical levels may provide insight into the mechanisms of production of congenital malformations and the feasibility of transplanting precursor cells into diseased tissues.